A valve leaking toxic and explosive ethylene oxide at Shell Chemical's Geismar complex Friday morning forced emergency responses from plant personnel for several hours to contain and shut the release, company officials and state regulators said.

But the leak, whose size was uncertain Friday, led to no reported injuries or exposures and no off-site impacts were detected, the officials said.

After the discovery of the leak about 8:05 a.m., a deluge system was used to reduce the hydrocarbon vapors at the Ascension Parish chemical plant along the Mississippi River while response personnel were dispatched, Shell spokesman Ray Fisher said in a statement.

Found in the rail yard, the leak prompted closures of stretches of Ashland and River roads around the plant as a precaution. But officials said the leak posed no threat to the parish's public schools in Dutchtown a few miles away or elsewhere, local and company officials said.

Shelter-in-place orders initially went out for workers at Shell and other nearby plants after the discovery of the leak, but at least some workers were being released by 1 p.m., deputies said.

Authorities issued an all-clear by Friday afternoon, opening up any remaining road closures, Fisher and deputies said.

Ethylene oxide, also known as "EO," is an explosive, flammable gas and a known human carcinogen at even minute levels over long-term exposure through inhalation, the U.S. Environmental Protection Agency recently found in a national assessment.

In acute exposures, the toxic gas can irritate the eyes, skin and respiratory system and cause nausea and vomiting.

Ethylene oxide is an intermediate most often used to create ethylene glycol, which is used in polyester fibers, fiberglass, antifreeze and some recyclable consumer plastic products, according to the American Chemistry Council.

The Shell plant makes 920 million pounds per year in ethylene oxide and 830 million pounds per year of different varieties of ethylene glycol, a company website says.

In May, Shell announced it was studying plans for a $1.2 billion expansion of its monoethylene glycol production and was expected to make a final decision this year. Shell finished an earlier olefins expansion in late 2018.

Lt. Col. Donald Capello, the Ascension sheriff's chief of criminal operations, said that by 1 p.m. Friday, officials were monitoring the air in the area as a team was preparing to check the suspected source of the leak.

By shortly before 4 p.m., Capello said the leak had been shut.

The size of the leak was unclear. Greg Langley, spokesman for the state Department of Environmental Quality, said agency officials on the scene never detected any concentrations of ethylene oxide. 

Opened in 1967, Shell's Geismar chemical complex is one of the oil major's three big manufacturing and refining facilities in Louisiana.

The more than 800-acre facility off River and Ashland roads has been a steady presence in Ascension's growing industrial corridor for decades.

Shell Geismar was the parish's top property taxpayer in 2018 and the fourth largest employer with 636 employees. The company held similar parish rankings in 2009, a parish audit says.

Email David J. Mitchell at dmitchell@theadvocate.com

Follow David J. Mitchell on Twitter, @NewsieDave.